Look in the panel
Carefully remove the cover of the main service panel, and take a look at the wiring. Looking at the neutral/ground bus bars, is probably the quickest and easiest way to tell.
You'll notice in this image, there is a mixture of both aluminum and copper wiring.
Look at the receptacle wiring.
Turn off power to the circuit you'll be inspecting at the breaker/fuse box. Then remove the cover plate from the receptacle. Use a flashlight to peek at the wiring attached to the receptacle. If you still can't tell. Remove the screws holding the receptacle in place, and carefully pull the receptacle out of the box.